Consider a library and the fact that books are loaned out to library members. Dates could be used heavily for the date a book was borrowed, the date the book was returned, and the due date for a book. Consider an entity type Loan that has attributes book identifier, member identifier, date borrowed, date due, date returned. What combination of attributes would be a key? Which attributes are key attributes? Which attributes are non-key attributes?

Key attributes in a relational database play a critical role in the organization and retrieval of data. They are essentially the cornerstone of a table's identity, serving to uniquely distinguish each record within that table. To understand key attributes, let's think of them as the unique identifier you would need to find a specific book in a massive library system.

As seen in the provided exercise, the 'book identifier' and 'member identifier' could be thought of as potential key attributes for the Loan entity. However, taking them individually would lead to ambiguity, since one member can have multiple loans, and each book might be loaned to several members over time. Consequently, the true power of key attributes emerges when we combine them to forge a super-key. In our library scenario, coupling 'book identifier', 'member identifier', and 'date borrowed' creates a unique identifier for every loan transaction—a combination that ensures each loan record is distinctly recognized.

When students are faced with determining key attributes, the exercise improvement advice suggests looking for the attributes that when combined, can ensure each record in the database can be uniquely identified. This is exactly what you would need if you were to look up the exact instance a book was checked out by a particular member.

Now let's shift our attention to non-key attributes. These are the details that add color to the data but aren't necessary to identify a record uniquely. Imagine attending a party; your name tag helps others recognize you (just like a key attribute), but the color of your dress or the style of your shoes (like non-key attributes) just provides more information about you.

In the context of our library example, 'date due' and 'date returned' are classic non-key attributes. They offer essential information about when a book is expected back and when it actually returns to the library. However, they don't have the unique identifying power of key attributes. Multiple loans could have the same due and return dates, and therefore, they can't serve as the sole means of uniquely identifying a loan record. Understanding this distinction is imperative for students, as emphasized in the exercise improvement advice, which suggests recognizing the difference between 'identifying' and 'descriptive' elements within a set of data.

Exploring the realm of entity relationships is akin to analyzing the web of connections within a social network where entities are friends, and the relationships are the interactions between them. In a database, entities relate to one another in various ways, and understanding these relationships is fundamental to organizing and querying data efficiently.

In our library database, we can envision a multitude of relationships. For example, a 'member' entity is related to a 'loan' entity by the action of borrowing a book. This relationship is typically one-to-many, as a single member can have multiple loan records. Moreover, a 'book' entity relates to a 'loan' entity when a member borrows it, here again showcasing a one-to-many relationship since a book can be involved in multiple loans over time.

Students are advised to always keep an eye on the way entities interact with one another. These relationships paint a comprehensive picture of the database schema and are integral to formulating queries and understanding the flow of data. In relational database terminology, recognizing the cardinality such as one-to-one, one-to-many, or many-to-many relationships is pivotal in database design, a crucial point made clear by the exercise improvement advice.